Typical designs for an infrared photodetector for a focal plane array comprise a “barrier” layer of constant alloy composition, whose composition is specifically chosen to produce a near-zero valence-band offset in order to obtain high-efficiency collection of photogenerated minority carriers. The prior-art devices are optimized for high-efficiency collection of photogenerated charge in the bulk of the device, but fail to provide mechanisms to minimize the effects of surface recombination/generation in the periphery outside the influence of the semiconductor contact, which is especially critical for devices utilizing small (<25 μm) pixels, with reduced junction areas, in practical focal-plane arrays. The effect of the surface recombination/generation in the periphery shows up in the dark current. In addition, these prior-art devices rely upon very wide bandgap AlAsSb alloys in the barrier layer, which are difficult to controllably dope, leading to variations in electric field in the narrow-bandgap absorber region of the detector structure.
Furthermore, prior art devices are not designed to ensure depletion-free, flat-band conditions in the absorber. In order to ensure flat-band conditions in the absorber, a compound barrier has to be incorporated such that the electric field in the absorber can be tailored by adjusting the doping in the barrier layer located adjacent to the absorber.